One of the most polymorphic and most widely used
enzyme loci in studies of population variation of honey bees has been the Mdh1
(malate dehydrogenase) locus. Its variation has been particularly useful,
because its allele frequencies differ greatly between different the lineages of
the honey bee. Therefore, Mdh variation has been extensively used in studies
about the spread of Africanized honey bees, or in studies on differentiation of
European lineages (Cornuet, 1982; Cornuet et al., 1986; Lobo et al, 1989;
Del Lama et al., 1990). However, as there is evidence that variation of
the Mdh1 locus can be interpreted as consequence of physiological adaptation to
different climates and thus may not necessarily reflect gene flow (Coelho and
Mitton, 1988; Nielsen et al., 1994, Cornuet et al., 1995),
phylogeographic conclusions should be drawn with caution, since the Mdh1 Locus
is obviously not selectively neutral.

Comparatively few loci are polymorphic in honey bees and there are no
fixed allelic differences between subspecies; therefore, variation within A.
mellifera consists exclusively of differences in allele frequencies between
populations. This limitation renders allozymes less suitable for the purpose of
identification of a small sampling or a single colony; however, the marker may
still be employed usefully in population studies. As an advantage, the method
is easy to use and scoring the data is comparatively unproblematic and
straightforward. In addition, the method is economical in setup and running,
since it does not require too much expensive laboratory equipment. The most
commonly studied enzyme systems and polymorphic loci, together with the most common
alleles and recommendations for staining are summarized in Table 5.